package main.java.sword2offer;

/**
 * <p>
 * 反转链表
 * </p>
 *
 * @author LovelyBigDuck
 * @date 2022/3/23 20:39
 */

public class S_206 {
    public class ListNode {
        int val;
        ListNode next;
        ListNode() {}
        ListNode(int val) { this.val = val; }
        ListNode(int val, ListNode next) { this.val = val; this.next = next; }
    }
    class Solution {
        // 1、迭代
        public ListNode reverseList(ListNode head) {
            ListNode prev = null;
            ListNode curr = head;
            while (curr != null) {
                ListNode next = curr.next;
                curr.next = prev;
                prev = curr;
                curr = next;
            }
            return prev;
        }
        // 2、递归
        public ListNode reverseList1(ListNode head) {
            if (head == null || head.next == null) {
                return head;
            }
            ListNode newHead = reverseList(head.next);
            head.next.next = head;
            head.next = null;
            return newHead;
        }
    }
}
